GIP and type 2 diabetes mellitus: In 1986, Nauck et al. described a reduced incretin effect in T2D patients, which explains the fact that the insulinotropic incretin hormones GIP and GLP-1 account for <20% of postprandial insulin response [22], while in non-diabetic individuals, both incretin hormones are responsible for 50-70% of the postprandial insulin response [23].